Ensure compatibility with Swift versions below 6

Author: ebariaux

Sources/ComplexModule/Complex+Codable.swift

@@ -12,7 +12,9 @@
 import RealModule
 
 // FloatingPoint does not refine Codable, so this is a conditional conformance.
+#if compiler(>=6.0)
 @_unavailableInEmbedded
+#endif
 extension Complex: Decodable where RealType: Decodable {
   public init(from decoder: Decoder) throws {
     var unkeyedContainer = try decoder.unkeyedContainer()
@@ -22,7 +24,9 @@ extension Complex: Decodable where RealType: Decodable {
   }
 }
 
+#if compiler(>=6.0)
 @_unavailableInEmbedded
+#endif
 extension Complex: Encodable where RealType: Encodable {
   public func encode(to encoder: Encoder) throws {
     var unkeyedContainer = encoder.unkeyedContainer()

Sources/ComplexModule/Complex+StringConvertible.swift

@@ -16,7 +16,9 @@ extension Complex: CustomStringConvertible {
   }
 }
 
+#if compiler(>=6.0)
 @_unavailableInEmbedded
+#endif
 extension Complex: CustomDebugStringConvertible {
   public var debugDescription: String {
     "Complex<\(RealType.self)>(\(String(reflecting: x)), \(String(reflecting: y)))"